1. Field of the Invention
The present invention relates to an image-forming process utilizing an electrophotographic method or an electrostatic recording method, and more specifically to an image-forming process developing an electrostatic latent image on an electrostatic latent image-carrier using a developer carried on a developer-carrier.
2. Description of the Related Art
A toner used for an electrophotographic process is generally produced by adding a coloring agent, a charge controlling agent, and releasing agent to a resin such as a polyester resin, a styrene-acrylic resin, an epoxy resin, etc., followed by melt-kneading the mixture, and after uniformly dispersing, by a grinding/classifying method of grinding the dispersion to definite particle sizes and further removing excessive course particles and fine particles using a classifier, but with the recent requirement of far increasing the image quality, it becomes necessary to further fining the particle sizes of a toner. Also, from the requirement of energy saving, a toner having a lower glass transition point and a lower softening point than the resin capable of fixing at a lower temperature has been used.
In color toners used for a full color copying machine or printer, it is necessary that the many color toners are sufficiently mixed in a fixing step, and also the color reproducibility and the transparency of an OHP image are important. To increase the color mixing property, color toners are generally formed with low-molecular weight resins having a sharp melting property as compared with a black toner. Consequently, there is a problem that an offset is liable to occur.
Hitherto, in a black toner, for obtaining an offset resistance at fixing, a wax having a high crystallizability and a relatively high melting point, such as polyethylene, polypropylene, etc., is used but in color toners for full color, the toners do not contain a wax for the reason of reducing the transparency of an OHP image, and a method of forming the surface of heat-fixing roller with a silicone rubber or a fluorine resin excellent in the releasing property to the toners, and further supplying a releasing liquid such as a silicone oil, etc., to the surface of the roller is employed for preventing the occurrence of offset.
The above method is very effective in the point of preventing the occurrence of the offset phenomenon of toners but there is a problem that an apparatus for supplying an offset prevention liquid become necessary. This is against the direction of small-sizing and light-weighing recently required, and also that is a problem that the offset preventing liquid is evaporated by heating to generate an unpleasant odor and contaminate the inside of the machine.
Accordingly, about color toners, particularly, color toners for full color, toners containing a low-melting wax have been investigated for realizing an oil-less system without reducing the characteristics required to color toners, such as a low-temperature fixing property, a sharp melting property, the reduction of the particle sizes of color toners, etc.
However, in color toners containing a wax, there are problems that the conveying property thereof is inferior, they cause staining a developer-carrier, and they cause image defects such as the density unevenness of images, etc. For example, in a method of carrying a two-component system developer having a toner and a carrier, developing an electrostatic latent image, and after transferring the toner image formed onto a transfer material, fixing the toner image to the transfer material, it is usually necessary that a proper and uniform developer layer is formed on the developer-carrier by a regulating member facing the developer-carrier. When a repeating imaging test is carried out using the two-component developer having the toner as described above by a developing machine having such a developer regulating member, the toner attaches the surface of the developer-carrier to stain the surface. By progressing the stain, the amount of the developer on the developer-carrier is changed to change the amount of the developing toner, whereby the density of the image formed is changed, a density unevenness in a solid image occurs, and the quality of the printed image is deteriorated.
The reason that the toner is liable to attach the surface of a developer-carrier has not yet been clarified but it is supposed that because the color toner containing a wax is more liable to be deformed than a conventional black toner and a color toner without containing wax and because the color toner contains many toners having small particle sizes and by the toners of the small particle sizes, the attaching property is relatively increased, the toner attaches to the surface of the developer-carrier as being rubbed thereto by a mechanical load to stain the surface. Also, it is considered to be a cause that the toner has a wax-exposed portions on the surface thereof.
In Japanese Patent Laid-Open No. H5-19632, for solving such a problem of staining the surface of the developer-carrier by the toner, a method of polishing the surface of the developer-carrier with large spherical particles such that the surface roughness Rz becomes a definite range is proposed. However, the method is not a system designed on the assumption of the developer using the toner made of a sharp melting resin and containing a low-melting wax and in the method, although in a repeating imaging durability test, there is a stain-preventing effect in the beginning, in the case of carrying out repeated imaging, staining cannot be sufficiently prevented.
In Japanese patent Laid-Open No. H8-15979, an image-forming method wherein the centerline average roughness (Ra) of the surface of the developer-carrier and the average spacing (Sm) of roughness peaks are define for improving the conveying property of a spherical toner is proposed. However, the spherical toner has a high fluidity and for conveying the toner, it is necessary to form large unevenness to some extent on the surface of the developer-carrier and thus in the case of using a random-shaped toner produced by a grinding/classifying method, the unevenness of the surface of the developer-carrier to too large to prevent sufficiently staining.
As described above, because the oil-less toners containing a wax are inferior in the conveying property and cause staining the developer-carrier, an image-forming process capable of using these toners in a practically using level has not yet been obtained.